Samson Servo 170 Power Amplifier cutting out

These good value for money home studio power amps must be one of the most popular home studio amps there is.   I guess it is because they look good in a home setting compared with traditional rack mounted gear.

Annoyingly the Samson Servo range does suffer from a common intermittent cutting out problem.  It manifests itself as the audio cutting out or just fading to a crackle on either channel occasionally, although the sound can break through if a loud passage in the music occurs.  Sometimes if you turn the volume it bursts through.  It’s a bit like the amp is strangling the audio, but if the audio is strong enough it can prevail.

When I first encountered this I thought it was just the spring loaded speaker wire clips not working too well, and it may be in some cases.  Then I thought it was just failing volume control pots as the Samson uses graduated pots I thought they might be prone to failure but no it was something else.  So upon further study I realised it was the relay that connects the amp to the speaker terminals.  Its job is to isolate the speaker from the amp during switch on; to protect your speakers.  The relay contacts have just become  dirty and that is what causes your audio to cut out.  Replacement of the relay is fairly straightforward and can be done within two hours.

So if your Samson is suffering and you would like to get it fixed please get in touch through my contact form.

Applies to Samson Servo 170 and many other models suitable relay is 24V coil, DPDT @ 8A rating available from usual places e.g. spiratronics

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Cornford

Had the pleasure of servicing a Hellcat combo this week.   A low serial number from around 2003 or 2004 built to the usual high  Cornford standard. Compared with your average production amp like Fender and Marshall it’s the little things that I notice on these amps that make them stand out.  For example aerospace PTFE wiring,  anti surge mains lead,  high audio quality coupling caps,  nyloc secured valve bases. All of these factors are likely to ensure that Cornford amps,  though not manufactured anymore  will be around for decades to come.

Hellcat
Channel switching problems can be cured by replacing the appropriate opto coupler. Original part is no longer easy to obtain but try subbing with VTL5C3 or 5C1.

Hurricane

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Hiwatt Hi Gain 50

Problem: excessive hum from speaker.
Solution :
Often caused by faulty capacitors not smoothing the rectified ac properly however in this amps case it was something else as well.  In this amp a number of op amps that are used for the effects send and return loop are powered by  a 30v supply.  It’s worth checking this as in ones case it had significant ripple caused by a number of minor components. Unusual because the current draw from the op amps would be minimal.

Fender DSP 90, champion 110 and similar

Nice little budget amps with a range of built in effects.  Seem to suffer from intermittent sound loss and cutting out.   The PCB are plagued with bad joints around ribbon cables, regulator  U10, and although I have not seen it the numerous large resistors and caps.

Also recently saw one making g a terrible humming or buzzing noise.  No signal was coming through at all. Current draw on the amp was about right so quickly able to link to open circuit power supply filter capacitors. Replaced, resoldered and glued into position and amp will be good as gold for another ten years minimum.

Vox Cambridge 30

Great looking vintage amps powered by tiny chip amp like you might find in car stereo but don’t let that put you off. Inside you have a pair of quality Celestion speakers,  twin channels clean and valve powered drive channel,  tremolo and reverb depending on model.

Construction wise it’s pretty poor inside with a budget Hi Fi style PCB.  Partly bad luck or design on korg ‘s part these amps have an amp failure disease named after them.  Cambridgitus effects this series of amps by reducing the output level in number of ways. Either temporarily, or gradually according to temperature or permanently at some random level. The tremolo effect may or may not stop working also.

In the temperature related version of Cambridgitus the level will creep down over 15 minutes or half an hour making you think you are losing your hearing.  With the permanent version the amp is sort of stuck on a lower level.  It is all due to components in the tremolo circuit namely the optocoupler. This optocoupler controls the level of the signal through a smooth light controlled sensor. Only problem is that korg picked the wrong type in its design and they just wrong all the time some from new, a suitable replace is the increasingly hard to find device called VTL5C3.

Laney VH100R

These monster heads are well liked by metal players for their high gain and awesome volume.

Well this one came to me like a crippled kitten, working one minute and then cutting out where volume dipped completely strangling the output. The cause? Well Laney do design great amps, in terms of tone, but due the their position in the mid tier price bracket it seems to mean that compromises sometimes have to be made. I totally get that but I would prefer them to limit the number of effects sockets instead of skimping on valve socket quality.

Yes in this case and other Laney models I have seen the valve socket connection with the valve pins been problematic. In this case there are two symptoms

1. crackles, pops and rumble noise
2. virtually total cut out

I am sure this is a quality issue and not a wear issue as I regularly come across forty year old Fender and Marshall’s where a repair can be achieved just by re-tensioning the pins but these brown Laney sockets just don’t seem to play ball and replacement is the only option. However the good news is that replacement sockets are cheap and do work.

Laney VC15R

Nice Diddy combos that have a massive sound despite their tiny size.

Issues I have seen inclue:
Output transformer
cut out after a while and the culprit turns out to be the tiny output transformer. If you google round you’ll find this isn’t such an isolated problem and if you have this then you are part of a growing club of Laney owners with, or seeking, I replacement transformers.

Laney, who support their customers very well supply replacements for the price of a couple of CDs. I chose to get one from another supplier that I have used for vintage transformers with great success livinginthepast. No doubt these guys know their stuff and I was really impressed with the quick service and great sound. For me the deciding factor on the Laney was that it is an uprated transformer slightly larger than the original so hopefully it won’t fail in the future. However these amps sound so good when cranked up it’s hard to resist pushing it a bit.

Valve bases
Some Laney models do seem to have fairly low quality valve bases and this can also present itself in an amp as cutting out, crackles and noise. It’s a shame as the difference between a cheap valve base and an expensive one is probably about the price of a bag of sweets.

Hohner Orgaphon 55

What an oddity. Is it a storage heater? Nope its a 212 hybrid combo from Germany and by date on some of the caps 1971.

The design inside is completely eccentric,  it feels more like a laboratory prototype than a rock amp. There are bus bars for the HT voltage, ground rail and little clusters of modular components featuring the BC109 transistors. The amp chassis lifts up like the bonnet of a car and access to the pots and controls is appalling.

Power stage is a pair of PL84 tubes driven by either an ecc83 or ecc808 depending upon the normal or bass channel.  Given that the amp is intended for organs the sensitivity of the input is not suited to guitar so for this client I have adjusted the gain stage to make it suited to the guitars 150mV output.

This is a great piece of musical history and so well made that even after 40 years there is barely anything other than a layer of dust on there to worry about.

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